This invention relates to the direct catalytic conversion of C.sub.1 -C.sub.4 oxygenates, e.g., alcohols, aliphatic ethers and industrial feedstreams containing these and other oxygenated lower aliphatic compounds, to low aromatic content distillate boiling range hydrocarbons. More particularly, the invention is concerned with the direct conversion of a feedstream containing at least one C.sub.1 -C.sub.4 oxygenate, e.g., methanol, and at least one light olefin, e.g., propylene, over the particular zeolite described infra to provide a mixture of low aromatic distillate range hydrocarbons useful as gasoline and/or distillate blending stocks.
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. These materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties. Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline silicates. These silicates can be described as a rigid three-dimensional framework of SiO.sub.4 and Periodic Table Group IIIA element oxide, e.g., AlO.sub.4, in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total Group IIIA element, e.g., aluminum, and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing the Group IIIA element, e.g., aluminum, is balanced by the inclusion in the crystal of a cation, e.g., an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of the Group IIA element, e.g., aluminum, to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given silicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. Many of the==zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite Z (U.S. Pat. No. 2,882,243); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite ZK-5 (U.S. Pat. No. 3,247,195); zeolite ZK-4 (U.S. Pat. No. 3,314,752); zeolite ZSM-5 (U.S. Pat. No. 3,702,886); zeolite ZSM-11 (U.S. Pat. No. 3,709,979); zeolite ZSM-12 (U.S. Pat. No. 3,832,449), zeolite ZS-20 (U.S. Pat. No. 3,972,983); zeolite ZSM-35 (U.S. Pat. No. 4,016,245); and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), merely to name a few.
The SiO.sub.2 /Al.sub.2 O.sub.3 ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with SiO.sub.2 /Al.sub.2 O.sub.3 ratios of from 2 to 3; zeolite Y, from 3 to about 6. In some zeolites, the upper limit of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is unbounded. ZSM-5 is one such example wherein the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is at least 5 and up to the limits of present analytical measurement techniques. U.S. Pat. No. 3,941,871 (Re. 29,948) discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added alumina in the recipe and exhibiting the X-ray diffraction pattern characteristic of ZSM-5. U.S. Pat. Nos. 4,061,724, 4,073,865 and 4,104,294 describe crystalline silicates of varying alumina and metal content.
In recent years, considerable research has been devoted to providing alternative sources and manufacturing routes for liquid hydrocarbon fuels in recognition of the fact that petroleum is a non-renewable resource and that petroleum-based fuels such as gasoline and distillate will ultimately become more expensive even should future supplies of petroleum temporarily increase.
The development of fossil fuel conversion processes has enabled the production of oxygenated hydrocarbons from coal, natural gas, shale oil, etc. Synthesis gas (CO+H.sub.2) is readily obtained from fossil fuels and can be further converted to lower aliphatic oxygenates, especially methanol (MeOH) and/or dimethyl ether (DME). U.S. Pat. No. 4,237,063 discloses the conversion of synthesis gas to oxygenated hydrocarbons using metal cyanide complexes. U.S. Pat. No. 4,011,275 discloses the conversion of synthesis gas to methanol and dimethyl ether by passing the mixture over a zinc-chromium acid or copper-zinc-alumina acid catalyst. U.S. Pat. No. 4,076,761 discloses a process for making hydrocarbons from synthesis gas wherein an intermediate product formed is a mixture of methanol and dimethyl ether.
Processes for the conversion of coal and other hydrocarbons to a gaseous mixture comprising hydrogen and carbon monoxide, carbon dioxide, etc., ("synthesis gas" or "syngas") are well known. A summary of the technology of gas manufacture, including synthesis gas, from solid and liquid fuels is provided in the "Encyclopedia of Chemical Technology", Edited by Kirk-Othmer, Third Edition, Vol. 11, pages 410-446, Interscience Publishers, New York, NY (1980), the contents of which are incorporated by reference herein.
It has recently been demonstrated that alcohols, ethers and carbonyl-containing compounds can be converted to higher hydrocarbons, particularly aromatics-rich high octane gasoline, by catalytic conversion employing a shape selective medium pore acidic zeolite catalyst such as H-ZSM-5. This conversion is described in, among others, U.S. Pat. Nos. 3,894,103; 3,894,104; 3,894,106; 3,907,915; 3,911,041; 3,928,483; and, 3,969,426. The conversion of methanol to gasoline in accordance with this technology (the "MTG" process) produces mainly C.sub.5 + gasoline range hydrocarbon products together with C.sub.3 -C.sub.4 gases and C.sub.9 = heavy aromatics. The desirable C.sub.6 -C.sub.8 aromatics (principally benzene, toluene and xylenes) can be recovered as a separate product slate by conventional distillation and extraction techniques. These light aromatics are also produced in a related process for converting methanol to olefins (MTO) as described in, amongst others, U.S. Pat. Nos. 4,011,278; 4,550,217; 4,513,160; and 4,547,616.
U.S. Pat. No. 4,439,409 discloses the use of zeolite termed "PSH-3" therein for the production of hydrocarbons from a feedstock containing methanol and/or dimethyl ether. In the conversion products analyses reported therein, the production of C.sub.1 -C.sub.2 gases was relatively high and notwithstanding patentees' characterization of the conversion product as one of low aromatics content, their analytic data establish that the products nevertheless contain amounts of aromatics exceeding those permissible for a quality distillate product.